This invention relates generally to a microbolometer focal plane array, and more particularly pertains to an improved readout technique for the microbolometer array.
Thermal infrared detectors are detectors, which operate by sensing the heating effect of infrared radiation. Thermal detectors generally do not need to be cooled below room temperature, which gives them an important practical advantage. Thermal infrared detectors that operate at room temperature have been known for 200 years, but recently the availability of integrated circuit and micromachining technology has greatly increased interest in this field. It is now practical to manufacture an array containing many thousands of thermal infrared detectors, which operates well at room temperature.
A bolometer is a thermal radiation detector that operates by absorbing incident electromagnetic radiation (typically infrared radiation), converting the absorbed infrared energy into heat, and then indicating the resulting temperature change in the detector by a change in its electrical resistance, which is a function of temperature. A microbolometer is a small bolometer, typically a few tens of microns in lateral size. Microbolometer infrared imaging systems are typically designed to be sensitive to long-wave infrared, typically in a wavelength range of about 8-12 micrometers. A two-dimensional array of such microbolometers, typically 120xc3x97160 microbolometers, can detect variations in the amount of radiation emitted from objects within its field of view and can form two-dimensional images therefrom. A typical array can have more than 80,000 microbolometers. Linear arrays of microbolometers may similarly be formed to form line images. In such large arrays of microbolometers, it is necessary to measure the resistance of all of the individual microbolometers in the array without compromising the signal to noise ratio of the microbolometers. Because it is impractical to attach thousands of electrical wires to such an array to measure all the microbolometer electrical resistances in the array, microbolometer arrays are typically built on a monolithic silicon called a xe2x80x9cread out integrated circuitxe2x80x9d (ROIC) which is designed to measure all the individual microbolometer electrical resistances in the array in a short time, called the xe2x80x9cframe time.xe2x80x9d The term xe2x80x9cframe timexe2x80x9d refers to a time in which a microbolometer array produces each complete picture or image of an object being viewed. The frame time is typically around {fraction (1/30)}th of a second, but it can be faster or slower than the typical time of {fraction (1/30)}th of a second. In order to allow the microbolometer array to respond adequately to time-dependent changes in the detected infrared radiation, the thermal response time of each microbolometer is typically adjusted, by power design, to be about the same value as the frame time.
A typical method used by the ROIC to measure the electrical resistance of all the microbolometers in the array is to apply a xe2x80x9cbias pulsexe2x80x9d of electrical voltage (or current) to each microbolometer in the array, and to measure a resulting signal current (or voltage). It is more common to apply a voltage bias pulse to each microbolometer in the array and to measure a resulting current signal from each microbolometer in the array during each frame time. In large arrays such as the one included in the ROICs, it is usual to apply such bias pulses to more than one microbolometer simultaneously, and to measure the resulting signal currents simultaneously. However, it becomes difficult to read each of the microbolometers in the array within the frame time. Therefore, it is advantageous to divide such large arrays into several smaller arrays to ease the reading process of large arrays. In such cases, each smaller array would be readout as if it were a separate array using its own data readout port and feeding data to a corresponding measurement circuit associated with each smaller array. The division of the large arrays need not be a physical division.
However, such a scheme of dividing the array into smaller arrays with each smaller array having its own measurement circuit to ease the reading of the larger array can produce undesirable deficiencies in the produced image. This is generally due to different drifts in the output signals of the smaller arrays induced by changes in different measurement circuit characteristics, such as offset voltage, offset current, and gain.
Therefore, there is a need in the art to design and operate a large array including multiple smaller arrays having their own data readout ports such that they do not produce the undesirable pattern artifacts in the produced image due to drifts in measurement circuits associated with each of the smaller arrays.
The present invention provides a technique to reduce undesirable pattern artifacts in an image produced by a microbolometer array including multiple smaller arrays. In one aspect of the present invention, this is accomplished by applying a bias pulse to each of the microbolometers in the multiple smaller arrays and measuring a resulting signal corresponding to the applied bias pulse for each of the microbolometers using multiple measurement circuits associated with the multiple smaller arrays during the frame time. Further, the technique requires applying one or more known bias pulses (calibration signals) to the multiple measurement circuitry associated with the smaller arrays during the frame time and measuring one or more resulting calibration signals corresponding to the applied one or more known bias pulses. Thereafter the technique requires computing an offset parameter for each of the multiple smaller arrays based on the corresponding measured one or more resulting calibration signals and correcting the measured resulting signal using the associated computed offset parameter to produce an output signal that significantly reduces the undesirable pattern artifact in the image.
Another aspect of the present invention provides a technique for reading out a large microbolometer array including multiple groups of microbolometers comprises using multiple measuring circuits to readout each of the groups of microbolometers. This is accomplished by applying a bias pulse during a frame time to each of the microbolometers in the group of microbolometers. The technique then includes measuring a resulting signal corresponding to the applied bias pulse during the frame time for each of the microbolometers in the groups of microbolometers using multiple measurement circuitry associated with each of the smaller arrays. Then the technique includes applying one or more calibration bias pulses during the frame time to the measuring circuitry associated with each of the groups of microbolometers. Further, the technique includes measuring one or more resulting calibration signals corresponding to the applied calibration bias pulses during the frame time. Thereafter the technique includes computing correction parameters for each of the groups of microbolometers and correcting the measuring resulting signal using associated computed correction parameters to produce an output signal that substantially reduces the undesirable image defects in the produced image.
Another aspect of the present invention is an infrared radiation detector apparatus for reducing undesirable deficiencies in an image produced by a microbolometer array including multiple smaller arrays. The infrared radiation detector apparatus comprises a microbolometer array including multiple smaller microbolometer arrays. The apparatus further includes a first timing circuit coupled to the array to apply a bias pulse to each of the microbolometers in the smaller arrays during a frame time. The apparatus also includes multiple measurement circuits coupled to the corresponding smaller arrays to measure resulting signals associated with each of the applied bias pulses during the frame time. Also included in the apparatus are multiple calibration circuits coupled to the corresponding measurement circuits. A second timing circuit coupled to the calibration circuits applies one or more calibration bias pulses during the frame time to each of the measurement circuits. Then the measurement circuits measure one or more resulting calibration signals corresponding to the applied one or more calibration bias pulses. The measurement circuits then compute correction parameters for each of the smaller arrays based on each of the measured one or more resulting calibration signals. The measurement circuits further apply each of the computed correction parameters to the measured resulting signal to produce an output signal that reduces undesirable deficiencies in the image produced by the microbolometer array.
Another aspect of the present invention is an infrared radiation detector apparatus for reducing undesirable pattern noise in an image produced by the microbolometer array including groups of microbolometers. The infrared radiation detector apparatus comprises a microbolometer array including groups of microbolometers. The apparatus further comprises a first timing circuit coupled to the array to apply a bias pulse to each of the microbolometers in the groups of microbolometers during a frame time. The apparatus also includes multiple measuring circuits coupled to the corresponding multiple groups of microbolometers to measure resulting signals associated with each of the applied bias pulses during the frame time. The apparatus further includes multiple calibration circuits coupled to the associated multiple measuring circuits. A second timing circuit is coupled to the multiple calibration circuits to apply one or more calibration bias pulses during the frame time to each of the multiple measuring circuits. The measuring circuits produce one or more resulting calibration signals corresponding to the applied one or more known bias pulses. A computing circuit coupled to the multiple measuring circuits compute offset and gain parameters and further applies the computed offset and gain parameters to the measured resulting signals associated with each of the multiple measuring circuits to produce an output signal that reduces undesirable pattern noise in the image produced by the microbolometer array.
Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.